One culprit could be higher mortgage rates, which have been trending upward. BankingMyWay's Weekly Mortgage Rate tracker pegs 30-year fixed mortgage rates at 4.31%, slightly ahead of last week's figure of 4.30% but still significantly higher than the 3.50% to 3.75% homebuyers were seeing last year.

Low inventory is another problem, as not enough Americans are putting their homes on the market this year. It turns out too many Americans feel they can't afford to stick a "for sale" sign in their front yard and move on with their lives.

According to research from Redfin, a Seattle real estate brokerage firm, only 49% of homeowners say they are in a "financial position to sell" their homes right now. That leaves 51% who aren't ready to sell, for a variety of reasons.

No liquidity. 19% of homeowners say their homes have low equity, meaning the homeowners owe more than 80% of the value of their home.

"No" to higher rates. 16% of homeowners are "locked in" with historically low mortgage rates. They don't want to give up those low rates by selling and moving on to a new home that comes with a higher mortgage rate attached to it.

The seven-year syndrome. 14% of U.S. residential properties were bought less than seven years ago. Redfin has found that homes typically sell seven years or more after their purchase.

Those concerns have led to a thin U.S. housing market in April, and buyers and their agents know it.